1. Field of the Invention
The present invention generally relates to the gallium-nitride based multi-quantum well light-emitting diodes and, more particularly, to the n-type contact layer structure of the gallium-nitride based multi-quantum well light-emitting diodes.
2. The Prior Arts
Gallium-nitride (GaN) materials can achieve a wide range of band gaps by controlling the GaN-based materials' compositions. As such, various colored light-emitting diodes (LEDs), especially those blue or purple light LEDs that require wide band gaps, can be produced using GaN-based materials. These GaN-based LEDs therefore have been the research and development focus in recent years.
Within conventional GaN-based LEDs, the active layers usually adopt a multi-quantum well (MQW) structure with GaN and indium-gallium-nitride (InxGa1-xN, 0≦x≦1) as potential wells. Photons are then generated through the recombination of the electrons and holes within the potential wells. Inside these conventional GaN-based MQW LEDs, the active layers are usually formed on top of an n-type contact layer made of n-type doped GaN.
Usually, in order to achieve low resistivity, the n-type GaN contact layer is heavily silicon (Si) doped (>1×1019 cm−3). However, it is observed that, during practical fabricating processes, the n-type GaN contact layer would be easily chapped and snapped, as the heavy Si doping causes incompatible lattice constants among the epitaxial layers of the GaN-based MQW LEDs, that, in turn, causes excessive stress to develop and accumulate. These undesirable results not only affect the epitaxial quality of the GaN-based MQW LEDs, but also add additional difficulties in the formation of the n-type ohmic contact electrode on top of the n-type GaN contact layer. In summary, these shortcomings would result in an inferior electrical characteristics or conductivity in the GaN-based MQW LEDs. In the worst case, the GaN-based MQW LEDs would be un-useable. Therefore, the GaN-based MQW LEDs containing this type of conventional n-type GaN contact layers, on one hand, require a higher operation voltage and, thereby, consume more powers. On the other hand, the GaN-based MQW LEDs would have a low yield rate, causing the production cost to rise.
Additionally, pin holes are easier to form in the heavily Si-doped n-type GaN contact layer, causing the semiconducting characteristics of the GaN-based MQW LEDs to deteriorate. Current leakage is also possible during the operation of the GaN-based MQW LEDs.
Accordingly, the present invention is directed to overcome the foregoing disadvantages of conventional GaN-based MQW LEDs according to prior arts.